Add docs for Swift usage to RCTBridgeModule.h

Summary:
Clarifies how to get a reference to RCTBridge from a Swift RCTBridgeModule.

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In writing native modules for Swift, I have run into issues where the documentation applies only to Objective-C and does not supply adequate information for how one might accomplish something in Swift.

This is a good example, where the `synthesize` method simply does not apply to Swift code.

[IOS][ENHANCEMENT][Base/RCTBridgeModule.h] Add documentation for Swift usage.
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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
Closes #18231

Differential Revision: D7262850

Pulled By: hramos

fbshipit-source-id: c6babb3edd786be8571eb49258d594f2fb99141d

React/Base/RCTBridgeModule.h

@@ -83,6 +83,7 @@ RCT_EXTERN void RCTRegisterModule(Class); \
  * to bridge features, such as sending events or making JS calls. This
  * will be set automatically by the bridge when it initializes the module.
  * To implement this in your module, just add `@synthesize bridge = _bridge;`
+ * If using Swift, add `@objc var bridge: RCTBridge!` to your module.
  */
 @property (nonatomic, weak, readonly) RCTBridge *bridge;